Lowering the dielectric constant of interlayer dielectric (ILD) materials for on-chip semiconductor interconnect structures is required to improve the performance for the next generation of integrated circuit (IC) devices. Historically, dense metal oxides such as, for example, silicon dioxide (SiO2), have been used as the ILD material in interconnect structures. While SiO2 is an excellent insulator with high modulus and hardness, and has a coefficient of thermal expansion (CTE) close to silicon, the dielectric constant (k) is approximately 4.0, which is too high for advanced generation interconnects. High-dielectric constants for ILD materials result in signal charging and propagation delays as well as increased transistor power budgets in the circuits that make up the IC. These circuit delays and power requirements are now the major issues relative to improving the performance of IC chips. As such, lowering the dielectric constant of the ILD material used for IC fabrication is a very important pursuit in current research and development.
Numerous “low-k” (k<4.0) materials have been developed and have been investigated for use as the ILD materials in interconnect structures. The lowest dielectric constants for fully dense dielectric materials typically are in the range of 2.7 to 3.3. To achieve dielectric constants significantly below 3.0 typically involves shifting from silicate to organic polymer materials, fluorination or methylation to lower dielectric polariziability, and/or lower the density of the insulator material such as by incorporation of controlled pores or voids into the insulator material. This latter approach, when applied to silicate or organosilicate materials, tends to produce elastically stronger but more brittle dielectric materials than the organic polymer analogs. The dielectric strength as well tends to be higher for the silicate-based materials. The higher the void volume in the dielectric material, the lower the dielectric constant. Unfortunately, as the void volume in the dielectric material increases, the mechanical properties of the dielectric material tend to degrade continuously. Ideally, one would like to introduce a very high level of void volume while maintaining good mechanical properties. Additionally, one would like to use an ILD material that has a CTE close to that of silicon. Since the mismatch in the CTE between the ILD and silicon chip is a primary mechanism for stress formation in the structure, the closer the CTE match between the ILD and silicon, the lower will be the stress introduced into the IC during thermal processing and use.
To achieve the required low-dielectric constant, high strength as well as closely match CTE, the incorporation of a controlled void volume into silicon dioxide would be ideal. The formation of controlled voids in silicon dioxide would require the introduction of a porogen material into silicon dioxide. Porogens are typically shaped organic molecules that are added to a system, and then thermally removed after the matrix is frozen to leave a void or pore. One problem associated with porogen inclusion into a matrix is that the porogen is typically very difficult to mix homogeneously in high volume into the dielectric matrix. As the required pore volume increases, the porogen and subsequent pore volume becomes inhomogenous due to such processes as agglomeration, precipitation, or coalescence. This can lead to “killer pores” that promote extrinsic failure at less stress conditions, such as low-field dielectric breakdown, and can render the dielectric material unusable in a high volume semiconductor application. In addition, these inhomogeneities can seriously degrade the mechanical properties of the porous dielectric. As such, porogen additions may not be the ideal methodology for the formation of low-k ILD materials.
In view of the drawbacks with prior art ILD materials, there is a continued need for providing new and improved ILD materials that have a dielectric constant that is lower than 4.0, yet are of high strength and have a CTE that closely matches that of silicon. Such ILD materials should be made without the use of a porogen so as to avoid the problems mentioned in the previous paragraph.